Low-frequency electric amplifier circuits



NovflZ, 1929. ,L RH. QRELEY l 1,735,750

LOW FREQUENCY ELECTRIC AMPLIFIER CIRCUITS A Phil/'p `6r caff] A ToRNEY.

Nov. 12,1929.

P. H. GREELEY- l 1,735,750 LOW FREQUENCY ELECTRI-C AMPLI-FI'ER'CIRCUITSFiled Jari. 1e. 192e s sheets-sheet 2 j Sec,

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DCP/aie Grid Bias I 4 ATTRNEY.

P. H. GREELEY 'LOW FREQUENCY ELECTRIC AMPLIFIER CIRCUITS Nov. 12, 1929.

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Patented Nov. 12,31-929i l PATENT 4OI=FICJE PHILIP H. GREELEY, oOFWASHINGTON, DISTRICT OF COLUMBIA Low-FREQUENCY ELECTRIC AMPLIFIERCIRCUITS` Application led January le, 192 8. Serial No.: 247,120.

i 10 quency electric currents a large number of times, as for example,the three or four hundred times commonly attained in the'audio portionof radio receivers, a plurality of thermionic tubes. are connectedV incascade. Such tubes are .most convenientlyoperated .from a commonsourcev of plate potential which usually is required to have a value inexcess of a hundred volts. As a source of high vplate potential,batteries have been used but to a considerable extent have been replacedby Y plate" current supply devices operating from `electriclightingcurrent. Such devices are A Vprovided with `a condenser'ofconsiderable capacity shunted across its output terminals intended toby-passalternating currents.'

'In the desire to obtain high quality audio amplification over the Wholemusical range, which is Aconsidered te extend from or 40 cycles persecond to '5,000 orr10,000 cps., suit'- 30 able units .for coupling theaudio amplifier i tubes have been developed. However, an

audio amplifier that will pass the low fre` quencies effectively oftengives trouble when the tubes-are'operated from a common source jsistance or .depending upon a condenser for lay-passing the alternatingcurrents'.v The rea actance of a condenser increases with decrease offrequency, and, at the lov'v frequencieathe by-pa'ss' condenserreactancei's not low enough to prevent coupling troubles between theseveral tubes working in the amplifier.

Several methods of 'reducing the magnitude Y sired results.

of .plate current having considerable relfrm a radio receiving set oraudio amplifier.

- hereinafter described and claimed.v

erated from a common source of plate po- 'pedanceiare employed, but suchtubes'waste current and are expensive. Partial separation of theindividual amplifier tube circuits by means of separate choke coils andcon` densers represent considerable expense if adequate values and sutlicient number are used.' However, tliese methods are successful onlyin reducing the magnitude of the disturbing effects. 1

An object of my invention is to provide a method and apparatus thatcanbe used to a balance out completelyv the coupling effect between eachpair oftubes.

Another object is tov provide a circuit Aand apparatus for an audioamplifier that will be free from peaks of excessive amplification causedbyregeneration.

A further object is'to provide an audio am-- plifier that is capable ofamplifying the lowest usefulA audio frequencies with perfect '70smoothness. l y

` Still another object is to reduce the number and values of 'suchauxiliary apparatus as chokes and by-pass necessary to obtain de- Anadditional object is to provide a radio receiving set in which thevacuum tube filaments may be heated by alternating currentWithsatisfactory Aelimination of alternating current hum. 80

Cth'er objects include.. the provision of means for reducingv thepossibility of undesirable pick-up of relatively high frequency currentsin an audio amplifier and means for gaining generally improvedperformance .'With these objects and other objects hereilia'lfvterdescribed in view,my invention-resides in the method, apparatusandcircuit '90- ,Y Referring to the drafvvings:

Figline 1 is a schematic diagram showing a circuit arrangementrfnr apair of tubes optential, .this arrangementbeing a form to yvhich mybalancing method is vreadily applicable. h

Figure 2 is a'diagrammatic representation of the circuit lconstantsof'Figure l adapted loo ing out of low frequency disturbance, such as'to illustrate the mathematical considerations involved.

Figure 3 isa schematic diagram for one practical audio amplifier circuitin which my invention may be applied.

Figurefl is a schematic diagram showing an alternating current filamentlighting circuitsuitable for usewith an audio amplifier of the typeshown in Figure and additional radio amplifying tubes.

Figure 5 is similar to Figure 4 except that provision is made forlighting the filaments of the detector and first audio tubes fromrectified and filtered current.

F igure is a schematic diagram showing coupling means for a pair ofradio amplifier tubeshsaid coupling means being adapted to balane outlow frequency disturbance in a radio frequency amplifier.

Figure 7 shows'scheinatically the elements of Figure 6 that areeffective in the balancmay be caused by `heating of amplifier tubefilaments on alternating current.

Figure 8 represents diagraminatically the relative direction ofinstantaneous signal and hum voltages that may be present in a radiofrequency amplifier of the type shown in Figure 6.

Figure 9 is a schematic diagram showing a modified form of my balancingmethod applied in a resistance or impedance coupled audio amplifier, and

Figure 10 is a sketch indicating a preferred arrangement of elements inan amplifier 'and showing means for practically eliminating undesirablepick-up of relatively high fre-` quency currents.

In the drawings, amplifying repeaters T1 and T2, here shownas-thefamiliar three element thermionic tubes, are coupled together byan audio. coupling unit. This unit may comprise aplate impedance orchoke coil L1, a plate circuit resistance R3, a coupling condenser C1, agrid impedance L3, a grid circuit resistance R5 and a connection througha grid biasbattery CB. A primary L2 of an audio transformer L2, L4 and aresistance R4 are shown connected in the plate circuit of tube T2.

Tubes T1and T2 have theirplates maintained at a positive potentiael withregard to their filaments by means of a platecurrent Supply device orbatteries connected across the terminals B minus and B plus. condenserC2 and a resistance R6 or their equi# A by-pass "'"valent are usuallyconnected across these B terminals. f l With regard to a signal voltageEs applied "familiar to those skilled in the artl acrossithe gridand'filament of tube T1, the amplifier operates in the normal mannerccrned with a detailed' consideration of vthe means used for amplifyingthesignal voltage Fis, the straight amplification of signal volt? agesbeing quite Well understood at the presi method, apparatus and circuitused to elimient time.

This invention resides mainly in the .tube of such amplifiers as areintended for the operation of loud-speaking reproducers. End audio tubescommonly have an instantaneous plate current variation of from 5 to 35milli amperes, and instantaneous potential fluctuations of a hundredvolts. In an amplifier in which a signal magnification of around 300times isattained, if as little as one three hundredth of the outputvoltage is fed back into the input, the amplifier may oscillate or maybe inoperative. In an audio amplifier that is capable of passing thelowest useful audio frequencies, it is a very difficult matter to keepthe alternating current voltage Erepresented as dueto the fictitiousgenerator Gr below an appreciable value.

According to my invention, I recognize the presence of an A. C.potential E and have cle-- vised ,a method, apparatus and circuit bywhich this potential E may be balanced out completely or to a degreethat is satisfactory for practical purposes. In using my method ofbalancing out .the effect of the A. C. potential E, it is mostpracticable to consider the amplifying tubes by pairs and so determinethe .circuit constants with respect to the A. C. plate impedance of .thefirst tube ofthe pair and the amplification attained between the outputof the first tube and the output of the second tube that the effect ofthe A. C. potential E may be balanced out completely or to the degreedesired.

Vith reference to Figures 1 and 2, it is well understood that theexternal impedance, Z1 (modified by Z3) and Z2 in thel plate circuits oftubes T1 and T2 should exceed the A. C. plate impedances yR1. and R2 ofsaid tubes at all frequencies which are to be amplified ef# fectively.In fact, the external impedance should be three or' more times greaterthan the tube plate impedance for practically disformer primary L2) andR4. These impedvoltage a E1 which is the same as the amances will belarge relative to the reactance of `by-pass condenserC2 and parallelresistance R at an luseful audio frequency. My

method of alan -ng may be made practically.

ndependentfl of the effective reactance -across the B terminals, so thatthis reactance or impedance will be considered only where its ef' fectis of importance.

Tov consider the theory of my method of balancing, suppose that the A.C. voltage E causes an effective voltage E1 to be applied across R1,thefilament to plate impedance of tube T1. E1 will cause an effectivevoltage E1 to be applied across the grid and lament of tube T2. With the'circuit arrangement of Figure l, E1 may be made nearly the same as E1for all useful audio frequencies to bepassed by 'the amplifier, thoughwith different circuit arrangements E1 may be lnade greater or less thanE1. E1 is then'amplified and reversed in phase by tube T2, giving theplified voltage a 1, considering a the amplification obtained in thecoupling unit and tube T2. It will be observed that the A. C. voltage Ealso causes an effective Voltage E2 to be applied across R2, thefilament to plate impedance .of tube T2, in addition to voltage E1applied across R1. In using my method of balancing, I arrange thecircuit and select constants so that aE1 is equal or approximately equalto E2 and Opposite in direction.' With this condition maintained eitherfor 'all audio frequencies o r for those low enough not to beeffectively by-passed by the 'condenser'and resistance across the Bterminals, it will be seen that an A. C voltage variation E will set upa zero -net effectiveV voltage across R2, the filament to plateimpedance of tube T2, and no current due to E will flow in the L2, L1.

The important'considerations in my method ofbalancing are the A. C.plate-resistances R1 and R2 of tubes T1 and T2 respectively,

the effective external impedances in the plate circuits of the tubes,and the amplification obtained in the coupling unit between the tubesand the amplification constant or n `of tube T2.

In considering the selection-of circuit constants for a practicalapplication of m invention, it appears desirable for the sa e ofsimplicity to eliminate Z3 from consideration. It is practicable andpreferable to make Z3 have a high impedance relative to R1 so that theeffective impedance of R1 in parallel with Z2 including the reactanc'eof condenser C1 will' be from 0.9 to 1 times the plate impedance R1alone at any useful audio frequency. lThis may be done readily by'making choke L3 have a high .inductance value or using a highresistance R5', orbo'th. .1n serles. The capacity of condenser C1 1spreferably made rather large, a capacity of plied by'R2 R1 Withoutprimary L2 of audio transformer' 0.5 mfd. being practicable. Thus, bymaking Z3 large, its effect Wouldnotbe of great i1nportance indetermining the balance necessary in the usual amplifier, though forexceptionally accurate balance,` the effect of Z3 would enterinto'computations. f

Then neglecting Z3 for simplicity, an A. C,

voltage E across the B terminals will cause a voltage E1 to be effectiveacross R1 in which case Where the resistanccs and rcactances appeal'-lng 1n the denominator are added as vectors. Similarly, the A. C.voltage E produces an effective voltage E2across R2, in which case,

The condition for balancing out the effect of voltage E is that aE1,which is the voltage numerator, so, to make E1I a fraction l/a (one overthe amplification) times E2, the

denominator in (3) will be made a times the denominator in (2). Asimple, yet reasonably accurate, expression for the condition of balanceis R2/R1 (R1-PZ1) =-.a .(R2-IZ2) (4) Where resistances and reactancesare added as vectors.

It will be noted that Equation (4) may be made to hold'independen't offrequency if the inductances and resistances entering into thevexpression for Z1 and Z2 are made to have suitable relative values.VHowever, it may be satisfactory to obtain my balancing effect only fora limitedband of frequencies, for` example, only such low audiofrequencies as lare passed by the amplifier yet are not effectivelyby-passed b the impedance across the B terminals. 'Ffbr low frequencies,the resistance components Rs and R1 of impcdances Z1 and Z2 are ofrelatively greater importance thanthe reactive components X1and X2.

" Although., it ispreferable to balance Van amplifier made in accordancewith the ideas.-

of this invention accurately, good results are obtainable in many caseswith, only anv approximate balance. Without understanding Atheprinciples explained in this specification, Sit would be unlikelythatone skilled in'the art would select tubes and circuitl constants andarrange al circuit giving even approximately the results. I obtain. Itistrue` that resistance coupled' units, impedance coupled units, andtransformers are used inaudio amplification, also various combinationsof these different units; yet, the selection ofl these different units'has rest-ed mainly upon the merits of the different types. with respectto" their ability to translate signal voltages,

.Adiagrii-m, tubes T1 and T2 have their circuits arranged 'as explainedabove with reference to the circuit of'Figure l. Tube T has its "inputconnected across the secondary L4 of transformer L2, L4, though acondenser C5 is preferably used instead of a direct connection to thetube filament so that a grid bias potential may be applied through asuitable resistance R7.- Tube T3 is coupled to an end audio tube T4 bymeans of a coupling condenser C3, plate circuit choke L5 and rcsistanceR8, and grid circuit choke LG and resistance Rf', condenser Cs beingused across the B terminals. Tube T4 has in its output circuit ka chokeL7 suitable for carrying direct current to the plate yet offering a highimpedance to alternating currents. C4 represents a. coupling condenserto the inductive load Ls which may be the Winding of a loudspeaker orother sound or signal reproducer. Any suitable source of plate potentialmay be connected across the terminals B plus and C minus, and asuitable' tapped resister R1 may be 'connected across 3the sameterminals sothat desired grid bias and plate potentials for thedifferent tubes may be obtained readyIn the amplifier circuit of Figure'3, tube T11 may be a detector in a radio receiving set and tube T2 thefirst audio amplifier. My methodrof 'balancing is of greatest importanceas applied to these two tubes, since the output'of these tubes isgreatly amplified. Itis preferable, also, to balancer-the circuits oftubes Ts'and T4 at least-partially.

In'additio'nto the balancing effect I obtain, I may use means to reducethe magnitude of plate voltage variations atsuitable points in thecircuits. Such means may comprise a choke coiland'by-.pass condenser asindicated by L]lo and C7, also, L and C. Although an audio amplifierthat isbalanced in accordance with my invention will be free of manytroubles without the use of such chokes as those indicated by L9 andL1", their use in some cases may be beneficial. A voltage variation oflargemagnitude across the B terminals of such tubes as are operated atmoderate or low plate voltages should preferably be'prevented. Radiofrequency tubes, detectors and rst audio tubes are usually operated atmoderate plate voltages and their plate supply may be filtered asindicated in' Figure 3.

It has been said that a voltage variation across a plate current supplydevice may be due to imperfect filtration but more commonly is caused bythe heavy current variations in the platecircuit of the last audio tubeof an amplifier. The arrangement shown in the plate circuit of tube T?,Figure 8, is desirable for the reasonthat the relatively largealternating currents are in the main choked out before being appliedacross the by-pass condenser C8 and resistance across the B terminals..Therefore, voltage fluctuations across the B terminals will not have asgreat magnitude as might otherwise be the case.

Although I have given the conditions that must .be satisfied to applylmy balancing method to an audio amplifier, it may be necessary to keepin mind some idea of the lowest or cutoff frequency of signal thatpasses through the amplifier. The cutoff frequency is, of course, notone definite frequency but rather the frequency below which theamplification falls rapidly. The cutoff frequency willbe determined bythe characteristics of the tube coupling devices. In the impedancecoupled stages shown in Figure 3, the frequency cutoff' may be setreadily at about the desired point by selecting values for the gridchokes L3 and L6 and by using condensers C and C1". The cutoff frequencyis of importance in that the balancing method needs to hold only down tothe cutoff frequency and -up to the frequencies not`efi'ective1ybypassed 'by a capacity across the plate current supplyunit. Y v

. In practical audio amplification, an amplifier that will pass notes aslow as 50 or 60 noticeable trouble may be constructed without greatexpense. Suchby-pass co d nsers as are necessary are not numerous ngreat in-.;I

value, values ,of about 2 microfafads'being suitable for mostv purposes.However an. amplifier passing notes down to 20 or 30 cps. can be madequite successfully and without great additional expense.

Tubes used in an amplifier built in accoz-dance with my invention may beof the usuali'general purpose types havingan amplification constant offrom 5 to, -10 and a plate impedance of from 10,000 to 25,000 ohms. Butbetter results may be obtained by using v power or semi-power tubes forTZ- and T4,

while T1 and T3 may be of the general purpose type or the high voltageamplification type. Four element tubes may be used with suitableconnections for the added element. Use of power tubes at T2 and-T4 willobtain best re- 4 sults from` an audio transformer or' loudspeakingdevice, and the low plate impedance of such tubes permits the conditionof balance to be satisfied with a greater effective direct currentplatevoltage on tubes T1 and T3.

I have described my method of balancing with regard to the platecircuits of amplifier` coupling units are particularly well suited tothe conditions existing in the different portions of the amplifier. TubeT1, if working as a detector, will usually have a high plate impedanceandthe external plate impedance should be exceptionally high; an 'audiotransformer will work best with its primary in the plate circuit of atube used as an amplifier and where the signal voltage is not ofgreatest magnitude, that is, most transformers will sh'ow overloaddistortion on strong signals; and-a dual or double impedance coupler isused as a coupling unit where the strongest si al t-ransfer takes place,the load capacity o a dual impedance of moderate cost being known to beexceptionally high. Dual and double impedance coupling units are knownin the art, the term dual7 being applied whenchokes L1 and L3 or L5 andL, Figure 3,'are wound on the same core and are inductively coupled, andthe term double, is generally used when the chokes are vwound onseparate cores and are not inductively coupled. yEither dual or doubleim 1 pedance coupling devices may be adapted to meet requirements incarryiny out my invention. The-relatively high resistances R3 and R8,Figure 3, do, however,change the action of such coupling devices as havebeen designed with the sole idea of functioning as 'audio couplingunits, such as the tuned impedance cou 1ers designed. to aord increasedampli cation near the resonant frequency. It is obvious that suitablecoupling units meeting the requirements of this invention may be.enclosed in a single case with four or fiv connecting terminals. l

Where an amplifier of greater power handling ability than-that aor'dedby the circuit of Figure 3 is desired, the well known pushrpullsystem.may be used. If tubes T3 and 1 are each replaced by a pair oftubes Working inthe, push-pull manner, my bal- 7 ancing system may beapplied to both halves of such an amplifier. l

In addition to eliminating trouble from plate supply voltage.variations, an amplifier constructed in accordance withthe principlesdisclosed herein `will prove advanta- -geous where the filaments of some0r all of the amplifier tubes arelig'hted on alternating current. It iswell-known that tubes lighted by alternating current tend to have an A.C.

variation in plate current which produces an audible hum in aloudspeaker or other reproducing device'. The causes of hum have beenstudied and tubes have been developed to reduce this hum to aminimum.The main hum is perhaps caused by variations of filament temperaturewhich result in a hum frequency of double that of the filament lightingcurrent.

If a pair of tubes connected in cascade and p0.

having their filaments operated on alternating current are consideredthe hum in both tubes will be in phase but the amplifiedv hum' from thefirst tube will oppose the hum in the second'tube lif the .circuitarrangement 5v is similar to that shown in Figure 1. If the` l humvoltage effective across the plate circuit of the first tube is keptrelatively low in magnitude so that when amplified it will approximatelyor exactly equal the hum effective across the plate circuit of thesecond tube, the hum voltage will cancel outin t-he out-putcircuit ofthe second tube. It is much easier to reduce the hum effect to a desiredA value than eliminate it entirely, variation of 105 the hum magnitudebeing -readily accomplislied either by changing the type of tube orchanging the tube operating conditions.

Suitable filament circuits for a sixtube radioreceiving set for A. C.filament heating are shown in Figures 4 and 5. In Figure 4,

tubes T10 and T11 are radio frequency amplifiers, T1 is a detector tube,T2, T3 and T4 are audio amplier tubes. These tubes are pref,-

erably of different types, each type being particularly suitable for itsspecificA purpose. It is common to employ tubes having low voltage, highcurrent filaments, withthe filaments themselves serving as electronemitters, for A radio and audio amplifiers operated' on A. C. 120

The detector is more commonly of the heater type, in which the filamentserves to heat an auxiliary electrode which serves as electron emitter;vgenerally, these differentl tubes require different filament voltageswhich are supplied from a step-down transformerhaving a plurality ofsecondary windings. The t last tube is preferably of the currentamplifier or power type. The detector and audio amphermbes will havetheir grid and plate me f the greatest importance.

tube T2 have their filaments in series and are lighted by direct currenteither from a batt`ery or from the plate `current supply device. Sincethe detector T1 and first audio tube T2 usually are required to operateonly on weak signal voltages, small tubes having low current filamentsare satisfactory and can. be lighted from the plate current supplydevice without placing a severe load on saidV device. Itis well knownthat the elimination of hum effects in the output of a vacuum tubeoperated as a detector is very difficult, yet is of Greater satisfactionis often obtained by operating the detector ondirect current rather thanalternating,

' 20 and where the detector isso operated, there is little added'diiculty operating the first audio tube from the same current source.The detector and first audio tubes, when operated from the same filamentcurrent supply,

are readily paired as shown in Figure 1 to eliminate undesirableeffects.

Although it might not seem' particularly important to eliminate humedects `present in a radio frequency amplifier, a careful considerationwill show that a slight modulation effect of a hum will be detected andgreatly amplified, therefore proving serious. As vis well known,modulation will be relatively greater on strong 4signals than Weaksignals,

and a powerful radio frequency amplifier will be most subject to themodulation effect of an A. C. hum. Most radio frequency amplifyingcircuits are not suitable for eliminating the effect of a low frequencyhum or l4o other disturbance, but the high frequency amplifying circuitdescribed in my co-pending application for U. S. Letters Patent,

Serial Number 150,57 6, filed November 241,."

1926, entitled Radio frequency amplifying apparatus, has been. found toeliminate or reduce the effect of an A. C. hum when the amplifier tubesare lighted on A.' C.

The essential circuit of my radio frequency amplifier is shown in'Figure 6. Since a low 5c frequenc alternating current or A. C. hum

Vis not a ected appreciably by the presence of the high frequency tuninginduetances, and since the capacity of tuning condenser C13 is smallrelative to the capacity' of the mutual condenser C12, the circuit isre-drawn in Figure 7, with these elements omitted. It will be seen thattubes T10 and T11 are capacig' tively coupled by condenser C11. If theradio frequency choke L11 has considerable resis- 50 tance or reactancetoward the hum frequency,

or'if a resistance R12 is provided', and if C11 has considerablecapacity relative to mutual condenser C12, an appreciable portion of thehum voltage'eifective across the plate circuit 05 of tube T1 will beapplied across the grid circuit constants may be select ed 'so that afraction equaling 1/V (amplification constant of tube T11) of the humvoltage effective across the plate circuit of tube T11- will be appliedacross its input. Since thephase of this hum voltage is reversed 'intube T11, the fractional hum voltage when amplified will equal andoppose the'normal hum. voltage effective across the output of tube T11,thereby eliminating the hum effect. In Figure 8 is shown the directionof the signal and hum voltages effective in'av circuit of the type shownin Figure 6. Sindicates the primary radio frequency signal voltage, Sthe secondary RF signal voltage and aS -the amplified RF signal voltageeffective across the plate circuit of tube T11. The hum volt- -age Heffective across the plat'ccircuit of tube T10 is reduced or attenuatedto H across the input to tube T11 and then amplified to cH which shouldequal and opposethe hum voltage H effective across the plat e circuitof'tube T11. y g

Although the elimination of A. C; hum is somewhat different from theelimination of the effects of an A. C. component in a common source ofplate potential, it will be seen that both may be accomplishedsimultaneouslyl obtainable with sources of plate and .filament currentthat ordinarily cannot be used satis# factorily.

The radio frequency amplifying circuit of lor a suitable reactance coil,offer convenient means for regulating the value of a low frequency A. C.voltage translated from tube T10 to tube T11. This low frequency A. C.voltage may be caused by A. C. heating of the filament of tube T10. Assuggested in my co-pending application referred to, a grid bias voltagefor the RF amplifier tubes may be supplied through a relatively highresistance R11 or'choke coil, or both in series. C011- siderableresistance at R11 appears to have a desirable leffect in that the radioamplifier 'tends to block and reduce the amplification of excessivelystrong signals and atmospheric strays or stat-ic.` Unusually steadyreceptionl of fading signals and relative freedom from noise have beenobtained with this type of amplifier, particularly when a plurality of-radio amplifier stages'are employed. Where more than one stage of thistype is used, it

.appears to be desirable to make the radio frequency cholesL11 inthedifferent stages somewhat different inv inductive value so that ltrouble from .undesirable resonance effects may be avoided. The resultsobtained by the luse of the radio amplier circuit of Figure 6 incombination' with an audio amplifierv circuit :similar to Figure 3 ina'com'plete radio receiving set with itspower supply have beenparticularly :satisfactory as toton'e quality, -even and steadyperformance at different Y resistance leak R39.

vradio frequencies covered by thetuned ciry cuits, and relative freedomfrom noise of any kind accompanying reception.

A modified application of my' balancing idea is shown in 'Figure 9,which represents a resistance coupled audio amplifier. T1 may be adetector having a grid condenser C31 and The use of a detector outputby-pass condenser C23 and a radio frequency choke 'L23 of suitablevalues is desirable. Plate circuit resistors R21, R22, R23, and R21having any suitable values, coupling condensers C21, C22, C23 and C2?having suitable capacities, and grid circuit resistors R31, R32 and R33having suitable resistances are provided for the several tubes. Suitableimpedance coils may be substituted for the resistances, or maybe used inaddition to the resistances as shown in the circuits of the end audiotube T* where chokes L33 and L34 are provided. In the case o f a powertube,which is preferable for last audio tube T3, the rela? tively largegrid swingand the high plate c urrent limit the circuit resistancepermissible, andl choke `coils having high A. C. impedance butyrelatively low D. C. resistance are pref` erable to pure resistances.

Comparing Figure 9 with Figures 1 and 2, T1 and T2 of Figure 9 may beconsidered. as a pair in which the external platerelsistance orimpedance Z1 is divided in two sections, one formed by R21 and one by aresistance R34, a luy-pass condenser C27 vbeing provided as indicated.Similarly, tubes T2 and T3 are paired, the extra resistance R35 andcondenser C23 being provided. Also, T3 and-T4 are paired, withresistance R36 and condenser C29 added. C30 is a by-pass condenseracross the f plate current supply. Practical values for condensers C27,C23, C22 and C30 are approxi- .mately 2 microfarads each, andresist-ances R32, R35 and R36 may each be of the order of 20,000 ohms.

If an A.

C. voltage effective across condenser' C28 is considered, a portion ofthis voltage will be effectiveV across the internal plate impedance oftube T2 and 'a much' smaller-portion of thisvoltage should be appliedacross the internalplate impedance of tube T1'. Instead of makingk theexternal plate resistance or vimpedance of tube T1.

greatly higher than resistance' or-impedance R22, the resistance Ritin'series with the reactance of condenser C27 1s employed so that v'thevoltage effectivel across C22 is'reduced as compared to that yappliedacross C28, the degree of' reduction depending upon the resistance R34andthe reactance of condenser C27 at the frequency of the voltage. Thevoltage effective across C22 is further reducedas applied across tlieinternal*plateresistance of tube T1.l The portion ofthe voltageeffective across condenser C23 appliedjacross the internal plateimpedanceof tube T1 should be only a fraction of the voltage applied.`across the internal plate impedance'of'tube T2, this fraction beingequal to or lessvthan the reciprocal of the yamplification gain betweenthe output circuits of tubes T1 and T2 for any frequency Within therange amplified.

It will be observed that the phase of a sigi nal voltage is 'reversed inthe step' from one tube to the following tube in the amplifier of Figurey9. Therefore, the voltage applied by an A. C. component E in the platecurrent supply across the internal plate impedance ebE l- ?l wy which isthe sum of thevoltages appliedv across the internal plate impedance ofthe third tube through the several couplings. Llkewise,

may vbe considered the sum of the vvoltages applied across the internalplate'impedance of. the fourth tube .through the several couphngs.Letters a, b and c represent the am- E CE 1 2 z 'y w lw, a', y and'zrepresent the attenuation factors indicating theportion of the ,fvoltage E .effective across the internal plate iin'- pedance of theseveral tubes.V In Equations 5, 6 and 7,.'if the fraction of E writtento plilication gain between successive tubes and i the left of the firstminus sign is always greater than the expression to the right of saidminus sign, the amplifier Will be free from regeneration.

In the amplifier represented in Figure 9, the approximate resistancevalues indicated may be found suitable for use With tubes giving anamplification of about 1().per stage at 30 cycles per second, condensersof 2 mfd. capacity being used at C2", C28, C20 and C30. Suchvcondenser-s in combination with resistors Ri, R35, and R3 form asectional filter attenuating an A. C. voltage of 30. cps. fre.- quencyto about one tenth in each section. Below this frequency the attenuationdecreases and the .coupling elements between the tubes should beselected so as to decrease the amplification between tubes accordingly.Tubes T1, T2 and T3 are operated as voltage amplifiers which may workeffectively with a plateicurrent of 1 milliampere or less. Consideringthese tubes to have plate currents of 1 millia'lnpere,the voltage'dropin R34 will be 20, in R35 will be 40, and in R3 will be 60. rIube T4 ispreferably a power tube having a plate current of about Q0n'1illiamperes,which would give a voltage drop of 80 in resistance R24.The voltages 135, 155, 195, and 255 represent practical values whichmight exist at the different points in this amplifier.

.It may be pointed out that a sectional filter of the type comprisingresistances R34, R35, and R35 with suitable condensers may be no moreexpensive to construct than a single filter for use on the detectorplate voltage supply alone, both filters giving the same attenuation atthe detector. An equivalent filter may be arranged with impedance coilsinstead of resistances. It is preferable to provide a separate filterand voltage divider for the plate potential for radio frequencyamplifier tubes, as indicated by choke L9 resistance R37 and condenserC26.

In the above description, possible phase angle differences have not beenconsidered, though it is desirable to. keep these in mind when designingan amplifier.

Since thls invention is intended to provide a generally .improvedamplifier, a method of reducing any tendency toward undesirable pick-upof relatively high frequency currents is represented in Figure 10. As 1swell known, high frequency pick-up or feed-back in an audio-amplifieroften results in the production of a high pitched whistle or hiss.Wherever it may be necessary to make relativelyr long leads connecting aunit such as the audio transformer L2, LfL 'of Figure 3 to tube socketsTS1 and TS2 and associated apparatus,-it is desirable to i sulate andtwist together the leads as indicated in Figure 10, thereby equalizingpick-up-in the going and returnwires By-pass condensers C5 and C7preferably are located close to ltheir respectiv'e tube sockets.4 Wherea volume control,

to be sufiicicnt to eut ofi' the amplification of any desirable highvaudio frequencies.

Although twisted leads are commonly employed for carrying alternatingcurrent power, as for filament lighting current in radio receivers, theuse of such twisted leads in-coinbination with by-pass condenscrssuitably located for carrying audio currents has not been generallyknown, yet improved rcsults are gained thereby.

In the following claims, the terms alternating currents or-potentialfluctuations are used without particular distinction between voltage audcurrent. It is, of course, understood that electric amplifiers areusually worked as voltage amplifiers and the consideration of voltagesmay be most import-ant except in the last or output stage. A fluctuatingdirect current may be considered as a steady direct current havingsuperimposed thereon alternating current. In this invention, thealternating currents or potential fluctuations considered are mainly ofan audio frequency range and particularly of a low audio range fromapproximately 20 to 40() cycles per second.'

Having thus described my invention what I claim is y 1. In electricsignal amplifying apparatus. having amplifying tubes connected incascade and operated from a common source of plate potential, means foreliminating from` the signal output of a pair oftubes undesired lowfrequency alternating currents present across the plate supply whichcomprises a circuit arrangement for said pair of tubes giving a reversalof phase of signal currents between their output circuits withimpedances of such relative inductive and resistive values withreference to said undesired alternating currents, between the plates ofsaid pair of'tubes and a common point of connection to the plate currentsupply as to make the plate circuits'impedance of the first tube.between itscathode and said common point as many times greater than theplate circuit impedance of the second tube between its cathode and saidcommon. point as the amplification factor between the output circuits ofsaid tubes. l

2. In electric signal amplifying apparatus having amplifying tubesconnected in cascade and operated'from a common source of platepotential, means for eliminating from the signal output of a pair oftubes undesired I5 first audio tube coup low ,fre uency alternatingcurrents present across t e plate supply which comprises a circuitarrangement 'for a pair of 'tubes giving a reversal of'phase of signalcurrents between their output circuits with impedances of such' relativeinductive and resistive values with reference tosaid.undesiredalternating current-s, between the plates of said pair of'tubes and a common point of connect-ion to lo the plate current supplyasto make the plate circuit impedance of the first tube between itscathode and said common point as many 'times reater than the platecircuit impedance o? the second tube between its cathode '15 and saidcommon point as the amplification factor between the output lcircuits ofsaid tubes, and means for cutting off signal ampliication below adesired minimum frequency comprising a circuit having low re.

go actance at frequencies below said minimum connected'across the inputof one of said tubes. 3. In electric signal amplifying apparatus havingampli ing tubes connected in cascade g5 and operated om a common sourceof plate potential, means for eliminating from the signal output of "apair of-tubes undesired low frequency alternating'currents presentacross .the plate supply which comprises a circuit .o arrangement orsaid pair of tubes giving a reversal *of phase of signal currentsbetween their output circuits with impedances of such relative inductiveand resistive values with reference to said undesired alternating cur-85 rents, between Athe plates of said pair of tubes and a common pointof connection to the plate current supply as to' make the plate circuitimpedance of the first tube between its cathode and said Vcommon pointas many o times greater than `the plate circuit imped anceof the secondtube between its cathode and said common point as the amplificationfactor between the output circuits of said tubes, in combination withsimilar means for eliminating from the signal out ut of a second pair oftubes similar undeslred low frequency alternating currents and means forcoupling the signal output of the first air of tubes to the input of thesecond pair o tubes.

o 4. In a radio receivin set having a detector tube and, audio ampli ingltubes connected in cascade andQ operated from. a common source of platepotential, means for filtering the plate ,currentv supplied to detectorand ed to detector separate from the plate current supplied to otherlaudio amplifyin tubes and means for elimlnatlngA from the signal outputofthe first audio tube undesired low frequency alternatin'gcurrents l0present across the separate plate current filter which comprisesaeircuit ,arrangement for detector and first audio tubes glvmg areversal of phase of' si alcurrent's between ``their`output circuitsw1th impedances of sucho u relative inductive and resistive values withreference to said undesired alternating currents, between the plates' ofthese two tubes and a common pointvof connection to the separate platecurrent filter as to make the plate circuit impedance of the detectortube between' 70 its cathode and said common point as many times greaterthan the plate circuit impedance o the first audio tube between itscathode'and said `common point as the amplifica-l -tiaon factor betweenthe output circuits .of said tu es.

5. In electric signal amplifying apparatus having amplifying tubesconnected incascade A and operated cfrom a common source of platepotential, means for eliminating from the slgnal output of a pair oftubes lundesired low frequency alternating currents present` across theplate supply which comprises a circuit arrangement for said pair oftubes giving a reversal of phase ofsignal currents between 35 theiroutput circuits with impedances having inductive reactance of valuecommonly used for signal fre uencies and such relative resistance valuesetween the plates of said pair of tubes and a common connection to theplate current supply as to make the plate circuit impedance withreference to'said undesired low frequencies, ofthe first tube betweenits cathode and said common point as many times `greater than the platecircuit impedance, with reference to said undesired low frequencies, ofthe second tube between its cathode and` said common point asthe'amplific'ation factor of said low frequencies between the outputcircuits .of said tubes. 10o 6. In an electric amplifier for audibletone frequencies having an output tube and a pre ceding pair of tubesconnected in cascade and operated from a common source of plateA poltential, means for eliminating detrimental 4105 plate circuit couplingeffects between the out? put tube and said preceding pair of tubescomprising an impedance connecting the plate of the output vtube to thevplate current supply and a separate filter section for the late currentof said preceding pair of tubes a apted to prevent appreciable coupling-between the output tube and said preceding air with l reference to allbut verylow au ible frequencies and a circuit arrangementfor saidpreceding pair of tubes vgiving a reversal of phase of signal currentsbetween their output circuits with -impedances having such relativeohmic values with reference to very low audible frequencies, between theplates of 120 said pair of tubes and a common ointof connection to saidseparate filter sectlon as to make plate circuit impedance of the firsttube of` saidy pair between its cathode and said. common-point as manytimes reater than lthe .125

late circuitl impedance -o'f etween its cathode and said ommon point `asthe amplification factor Ifor id very low audible frequencies betweenthe output circuits of said pain of tubes, thereby liminati'ng 13e I'amplification which consists in arranging cirplifying tubes connected incascade and 'op- 'erated from a common course of plate potencuits-forsuccessive tubes giving a reversal of phase of signal currents in theiroutput circuits and making the effective voltage applied across theinternal plate impedance of apreceding tube through 'plate circuitcoupling a fraction, which is less than one over the amplification gainbetween the output circuits of said successive tubes, of the effectivevoltage applied across the internal plate impedance of a following tubethrough plate circuitcoupling, for frequencies within the desired rangeof audible tone frequencies.

8. In electric amplifying apparatus for a range of audible tonefrequencies having amtial, a circuit arrangement for successive tubesgiving a reversal of phase of signalcurrents in their output circuitsand means for reducing the effective voltage applied across the internalplate impedance of a preceding tube through plate circuit coupling to avalue which is less than a fr-action, equalling the reciprocalofamplification gain `between the output circuits of said successivetubes, of the effective voltage appliedv across the internal plateimpedance of a following tube through plate circuit coupling, forfrequencies within the desired range of audible tone frequencies.

9. In an electric amplifier for low frequency alternating currentshaving amplifying tubes connected in cascade and operated from a commonsource of plate potential, an electric circuit arrangement for a pair oftubes comprising a relatively high impedance coupling unit connectingthe alternating current output of the first' tube to the input of thesecond tube and a low frequency transformer having relatively lowvprimary impedance connected in the output circuit of the second tube,the ratio of the coupling impedance and transformer primaryimpedancebeing taken with relation to the internal plate impedances ofsaid tubes and the amplificavtion gain between their output circuits,and

a second electric circuit arrangement for -a second pair of tubescomprising a relatively high impedance coupling unit` having rela-`tively high signal load capacity connecting the signal current output ofthe first'tube of second pair to the input of the second tube of saidpair and an output coupling circuit.

of relatively low impedance for this last tube,

the input for they second pair of tubes being j connected to secondaryof the transformer undesired alternating currents present across theplate current supply which comprises an output device having impedancelcoupled through a directl current blocking4 condenser between the outputterminals of the second amplifying tube and an external plate impedanceof highervalue.y connecting th'eplate of` said tube to the common sourceof plate po tential and a lcoupling circuit between the outputof'the'first tube and input of the second tube having an impedancecoupled throughva condenser between the output ter-- minals of saidfirst tube and an external plate impedance of higher value connectingthe plate of said firstrtube to said plate current supply, said externalplate impedance of the first tube being pf such value with reference tolow frequencies relative to theexternal plate Vimpedance of the' secondtube as to make the sum of tlie external and internal impedances of thefirst' tube as many times as great as the 'sum of the external andinternal impedances of the second tube as the amplification factorbetween the output circuits of said tubes.

l1. In combination, an alternating current operated plate currentsupplyl device and radio receiving set having operating circuits foradetector tube and a first audio amplifier tube connected in cascade andfollowed by a ioo power amplifier, means for filtering the plate"current supplied to said detector tube and audio amplifier tube.separate from thatvsupplied to said power amplifier, means for reducingunwanted low frequency electric cur# rents in thesignal output of theaudio amplifier tube comprising a circuit arrangement for detector andaudio tubes giving a: reversal of phase of signal'currents between theiroutput circuits with more than six times as much re sistance effectivein the external plate circuit of the detector tube as the effectiveresistance in the externalplate circuit of said audio amplifier tube.,the power amplifier circuit being balanced to eliminate from its out utunwanted currents, and means for coup 'ng the signal output of saidaudio tube to said power amplifier.

` In testimony whereof, I yhereunto affix my

